Problem: What is the greatest common factor of $6$ and $15$ ? Another way to say this is: $\operatorname{gcf}(6, 15) = {?}$
The greatest common factor is the largest number that is a factor of both $6$ and $15$ The factors of $6$ are $1$ $2$ $3$ , and $6$ The factors of $15$ are $1$ $3$ $5$ , and $15$ Thus, the greatest common factor of $6$ and $15$ is $3$. $\operatorname{gcf}(6, 15) = 3$